Magnetic recording medium and process for producing the same

ABSTRACT

The present invention provides a magnetic recording medium having a thinned magnetic layer, excellent electromagnetic conversion characteristics and an excellent durability, and a process for producing the same.  
     A magnetic recording medium comprising a lower non-magnetic layer containing at least a carbon black and a radiation curing type binder resin on a non-magnetic support and an upper magnetic layer having a thickness of 0.30 μm or less on the lower non-magnetic layer, wherein the upper magnetic layer contains at least a ferromagnetic powder, a binder resin and an abrasive having a Mohs hardness of 6 or higher and a smaller average particle size than the thickness of the upper magnetic layer. The thickness of the upper magnetic layer is, for example, 0.05 to 0.30 μm. The average particle size of the abrasive is, for example, 0.01 to 0.2 μm.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a magnetic recording medium, andmore specifically to a magnetic recording medium excellent inelectromagnetic conversion characteristics and durability. The presentinvention also relates to a process for producing the magnetic recordingmedium.

[0003] 2. Disclosure of the Related Art

[0004] Some magnetic recording media comprising a lower non-magneticlayer and an upper magnetic layer in this order formed on a non-magneticsupport are disclosed in the prior art.

[0005] For example, Japanese Examined Patent Publication No.1-30221(1989) discloses, “a magnetic recording medium formed by forminga non-magnetic underlayer on a substrate, and then forming a magneticlayer, characterized in that the underlayer is formed by using aradiation-curing coating material which contains at least two kinds ofcompounds selected from (A), (B) and (c) described below:

[0006] (A) radiation-curing compound having two or more unsaturateddouble bonds and a molecular weight of 5000 or higher, preferably 8000or higher,

[0007] (B) radiation-curing compound having one or more unsaturateddouble bond and a molecular weight of 400 or higher and lower than 5000,preferably 6000 to 3000,

[0008] (C) radiation-curing compound having one or more unsaturateddouble bond and a molecular weight of lower than 400; and by irradiatingwith radiation to the coated substrate.”

[0009] However, since there has hardly been referred to a composition ofthe magnetic layer in the Publication, it is difficult to obtain themagnetic recording medium having satisfactory coating layer properties.

[0010] Japanese Patent Publication (B2) No. 2566085(1996) discloses, “amagnetic recording medium comprising a non-magnetic layer based on anon-magnetic powder and a binder resin and a magnetic layer based on aferromagnetic powder and a binder resin in this order formed on anon-magnetic substrate, characterized in that the magnetic layer has athickness of 1.0 μm or less and contains an abrasive material which hasa Mohs hardness of 6 or higher and a larger average particle size thanthe thickness of the magnetic layer.” And the same Publication discloses“a process for producing a magnetic recording medium, characterized bypreparing respectively a coating liquid for a lower non-magnetic layerincluding a non-magnetic powder dispersed into a binder, and a coatingliquid for a magnetic layer including a ferromagnetic powder and anabrasive material having a Mohs hardness of 6 or higher and a largeraverage particle size than a dried thickness of the magnetic layerdispersed into a binder; coating the coating liquid for the non-magneticlayer onto the non-magnetic substrate to obtain a non-magnetic layer;and coating the coating liquid for the magnetic layer simultaneouslywith or successively to coating of the lower non-magnetic layer whilethe lower non-magnetic layer obtained is in a wet state so that a driedthickness of the magnetic layer can be 1 μm or less.” However, in thisproducing method, because the upper magnetic layer is coated while thelower non-magnetic layer is in a wet state, an interface between theupper magnetic layer and the lower non-magnetic layer becomesnon-uniform and this results in output variations. Also, as the particlesize of the abrasive is larger than the thickness of the magnetic layer,problems in running durability arise.

[0011] Japanese Patent Publication (B2) No.2922771(1999) discloses amagnetic recording medium having at least two coating layers on onesurface of a non-magnetic support, wherein the uppermost of the coatinglayers is a magnetic layer having a thickness of 0.2 to 0.5 μm and themagnetic layer of the uppermost layer includes Al₂O₃ particlescontaining an inorganic substance of an average particle size of 0.1 to0.3 μm. And, the same Publication discloses forming the magnetic layerby wet-on-wet coating method. According to the examples of the samePublication, in both the upper magnetic layer and a lower non-magneticlayer, a thermoplastic resin is used as a binder and the magnetic layerhaving a thickness of 0.4 μm is formed by wet-on-wet coating method.

[0012] However, the same Publication does not particularly consider acase of a thinner thickness of the upper magnetic layer such as 0.30 μmor less, and if such an upper magnetic layer having a thinner thicknessis formed by wet-on-wet coating method, an interface between the uppermagnetic layer and the lower non-magnetic layer becomes non-uniform andthis results in output variations.

[0013] In recent years, a magnetic layer has shown a marked trend towarda thinner layer, a higher filling and a higher durability from a demandfor magnetic recording media that have an increased recording density.

SUMMARY OF THE INVENTION

[0014] Accordingly, it is an object of the present invention to providea magnetic recording medium which solves problems of the prior art, hasa thinned magnetic layer, has excellent electromagnetic conversioncharacteristics and has an excellent durability. It is a further objectof the present invention to provide a process for producing the magneticrecording medium.

[0015] The present inventors worked assiduously and has found that evena thickness of an upper magnetic layer is as thin as 0.30 μm or less, amagnetic recording medium which has excellent electromagnetic conversioncharacteristics and an excellent durability can be obtained by using aradiation curing type binder resin as binder of a lower non-magneticlayer and by using an abrasive having a Mohs hardness of 6 or higher anda smaller average particle size than the thickness of the upper magneticlayer, and has completed the present invention.

[0016] That is, the present invention relates to a magnetic recordingmedium comprising a lower non-magnetic layer containing at least acarbon black and a radiation curing type binder resin on a non-magneticsupport and an upper magnetic layer having a thickness of 0.30 μm orless on the lower non-magnetic layer, wherein the upper magnetic layercontains at least a ferromagnetic powder, a binder resin, and anabrasive having a Mohs hardness of 6 or higher and a smaller averageparticle size than the thickness of the upper magnetic layer.

[0017] The thickness of the upper magnetic layer is generally 0.05 to0.30 μm. The average particle size of the abrasive is smaller than thethickness of the upper magnetic layer, and is generally 0.01 to 0.2 μm.

[0018] A centerline average roughness (Ra) of the upper magnetic layersurface is preferably 1.0 nm≦Ra≦8.0 nm.

[0019] The abrasive preferably contains two or more kinds of abrasiveswhich have different average particle sizes to each other.

[0020] Also, the present invention relates to a process for producing amagnetic recording medium which comprises:

[0021] preparing respectively a lower non-magnetic layer coatingmaterial including at least a carbon black dispersed into a radiationcuring type binder resin, and an upper magnetic layer coating materialincluding at least a ferromagnetic powder, and an abrasive having a Mohshardness of 6 or higher and a smaller average particle size than athickness of an upper magnetic layer dispersed into a binder resin,

[0022] applying the lower non-magnetic layer coating material onto anon-magnetic support, drying the coating material, and carrying outsmoothing treatment of and irradiating with radiation to resulting layerto form a lower non-magnetic layer, and then

[0023] applying the upper magnetic layer coating material onto the lowernon-magnetic layer, drying the coating material, and carrying outsmoothing treatment of resulting layer to form an upper magnetic layer.

[0024] Furthermore, the present invention relates to the magneticrecording medium produced by said process.

[0025] According to the present invention, a magnetic recording mediumhaving a thinned magnetic layer and being excellent in electromagneticconverting characteristics and in durability is provided.

DETAILED DESCRIPTION OF THE INVENTION

[0026] Now, the specific configurations of the present invention will bedescribed in detail thereafter.

[0027] The magnetic recording medium of the present invention has atleast two coating layers on a non-magnetic support, that is, a lowernon-magnetic layer and an upper magnetic layer having a thickness of 0.3μm or less formed in this order, and on the other surface of thenon-magnetic support, a back-coat layer is formed as required. Further,in the present invention, a lubricant coating layer, and/or variouscoating layers for protecting the magnetic layer may be formed on theupper magnetic layer as required. Also, on the surface of thenon-magnetic support, on which the magnetic layer is to be formed, anundercoat layer (adhesion facilitating layer) may be formed forimproving adhesion between the coating layer and the non-magneticsupport.

[0028] [Lower non-magnetic layer]

[0029] The lower non-magnetic layer contains at least a carbon black anda radiation curing type binder resin.

[0030] By allowing the lower non-magnetic layer to contain carbon black,a lubricant can be held in the layer. As a result, the content of thelubricant on the upper magnetic layer surface is able to be easilyadjusted to a desired range. In the present invention, as the thicknessof the upper magnetic layer is as thin as 0.3 μm or less, it isdifficult for the upper magnetic layer only to contain a sufficientamount of lubricant, and the carbon black of the lower layer is anessential component. Also, the carbon black of the lower non-magneticlayer has an effect to lower the surface electric resistance of theupper magnetic layer as well as an effect to reduce the lighttransmittance.

[0031] Examples of carbon black contained in the non-magnetic layerinclude furnace black for rubber, thermal black for rubber, black forcolor, acetylene black, etc. Preferably the carbon black has a specificsurface area of 5 to 600 m²/g, a DBP oil absorption of 30 to 400 mL/100g, and a particle size of 10 to 100 nm. The carbon black which can beused may be referred specifically to “Carbon Black Almanac,” compiled bythe Society of Carbon Black.

[0032] For the non-magnetic layer, various inorganic powders may be usedin addition to carbon black, and for example, needle-like non-magneticiron oxide (α-Fe₂O₃), etc. may be used. However, the use of sphericalsuper-fine particle iron oxide can achieve high dispersibility, and thefilling ratio of particles in the non-magnetic layer can be increased.Consequently, the surface smoothness of the non-magnetic layer itself isimproved, and as a result, the surface smoothness of the magnetic layeris improved, and the electromagnetic conversion characteristics areimproved. In addition to these, various non-magnetic powders such asCaCO₃, titanium oxide, barium sulfate, α-Al₂O₃ may be used.

[0033] The composition ratio of carbon black to inorganic powders ispreferably 100/0 to 75/25 by weight ratio. When the composition ratio ofinorganic powders exceeds 25 parts by weight, problems occur in surfaceelectric resistance.

[0034] The composition ratio of carbon black in the lower non-magneticlayer is 35 to 90% by weight and preferably 40 to 85% by weight. Whenthe composition ratio of carbon black is less than 35% by weight, thedesired amount of lubricant is unable to be held. On the other hand,when carbon black is used in the ratio of 90% by weight, the sufficientamount of lubricant is able to be held, and when carbon black is usedmore than 90% by weight, the ratio of the binder resin in the lowernon-magnetic layer lowers and sufficient strength of the coating layeris unable to be obtained.

[0035] The binder resin which is able to be used for the non-magneticlayer is limited to the radiation curing type binder resin. With athermoplastic resin and a thermosetting resin which have been usedconventionally, the raw roll coated with non-magnetic layer must beplaced in an oven for a long time (for example, at 70° C. for 2 to 48hours) and cured in order to obtain sufficient coating layer properties.This causes problems of deformation of the non-magnetic coating layerdue to tightened winding or deteriorating surface smoothness of thenon-magnetic coating layer, not to mention the time and labor requiredin the manufacturing process.

[0036] In the manufacturing method for coating the upper magnetic layerwhile the lower non-magnetic layer is still wet, as disclosed inJapanese Patent Publication No. 2566085, the interface between the uppermagnetic layer and the lower non-magnetic layer becomes non-uniform, andthis results in output variations.

[0037] In order to eliminate defects like these, in the presentinvention, the radiation curing type binder resin is used for the binderresin of the lower non-magnetic layer, the lower non-magnetic layercoating material is applied on a non-magnetic support, dried, andsmoothened; then, radiation is irradiated, the three-dimensionalcrosslinking is allowed to occur; and then, the upper magnetic layercoating material is applied on the lower layer, and thereby a goodresult is able to be obtained. According to this process, since thelower non-magnetic layer has the three-dimensional crosslinking when theupper magnetic layer is formed, it is not subject to swelling caused byan organic solvent. Consequently, because the magnetic coating materialis able to be applied directly to the lower non-magnetic layerimmediately after the lower non-magnetic layer is formed, continuationand simplification of the process is able to be achieved.

[0038] The radiation curing type binder resin used in the presentinvention is the resin containing one or more unsaturated double bondsin the molecular chains which generates radicals by radiation and curesby crosslinking or polymerizing.

[0039] Examples of radiation curing type binder resin include vinylchloride type resin, polyurethane resin, polyester resin, epoxy typeresin, phenoxy resin, fiber type resin, polyether type resin, polyvinylalcohol type resin, and many other resins. Of these, vinyl chloride typeresin and polyurethane resin are typical, and it is preferable to usethe both in combination.

[0040] The radiation curing vinyl chloride type resin is synthesized bymodifying a vinyl chloride type resin as raw material to radiationfunctional type resin. For the vinyl chloride type resin as rawmaterial, the vinyl chloride type resin of which vinyl chloride contentis 60 to 100% by weight, and preferably 60 to 95% by weight inparticular. Examples of such vinyl chloride type resin include vinylchloride-vinyl acetate-vinyl alcohol copolymer, vinylchloride-hydroxyalkyl (meth) acrylate copolymer, vinyl chloride-vinylacetate-maleic acid copolymer, vinyl chloride-vinyl acetate-vinylalcohol-maleic acid copolymer, vinyl chloride-hydroxyalkyl (meth)acrylate-maleic acid copolymer, vinyl chloride-vinyl acetate-vinylalcohol-glycidyl (meth) acrylate copolymer, vinyl chloride-hydroxyalkyl(meth) acrylate-glycidyl (meth) acrylate copolymer, vinylchloride-hydroxyalkyl (meth)acrylate-allyl glycidyl ether copolymer,vinyl chloride-vinyl acetate-vinyl alcohol-allyl glycidyl ethercopolymer, and the like. Especially, a copolymer of vinyl chloride and amonomer which contains epoxy(glycidyl) group is preferable. And, theaverage polymerization degree of the copolymer is preferably 100 to 900,and more preferably 100 to 600.

[0041] Furthermore, in order to improve dispersibility, it is preferableto introduce polar groups such as —SO₄Y, —SO₃Y, —POY, 1'PO₂Y, —PO₃Y,—COOY (Y represents H or alkaline metal), —SR, —NR₂, —N⁺R₃Cl⁻(Rrepresents H or hydrocarbon group), phosphobetaine, sulfobetaine,phosphamine, sulfamine, and the like into the copolymer by optionalmethods as required. Also, to improve heat stability, an introduction ofepoxy group is preferable.

[0042] As methods for modifying the vinyl chloride type resin toradiation functional type resin, for resins having hydroxy group orcarboxylic acid group, a modifying method by reacting the resin with acompound having (meth) acrylic group and carboxylic anhydride ordicarboxylic acid, a modifying method by reacting the resin with areactant (adduct) of tolylene diisocyanate (TDI) and 2-hydroxyethylmethacrylate (2-HEMA), and a modifying method by reacting the resin witha monomer which has one or more ethylene unsaturated double bond and oneisocyanate group in a molecule and has no urethane link in molecule aretypical. Of these methods, the third one is excellent in modificationeasiness, and dispersibility and physical properties after modification,therefore the modification is preferably carried out by the thirdmethod. Said monomers include 2-isocyanate ethyl (meth) acrylate, andthe like.

[0043] Acryl groups or methacrylic groups in the binder moleculepreferably exist 1 to 20, and more preferably 2 to 15 on average permolecule.

[0044] The radiation curing polyurethane resin is an urethane resinhaving at least one acrylic bond in its molecule, namely a polyurethaneacrylate compound formed by bonding to a compound containing acrylictype double bonds through urethane bond.

[0045] The acrylic type double bond mentioned here represents a residue(acryloyl group or methacryloyl group) of acrylic acid, acrylic ester,acrylic amide, methacrylic acid, methacrylic ester, methacrylic amide,and the like.

[0046] As the compound containing acrylic type double bonds (A),mono(meth)acrylates of glycol such as ethylene glycol, diethyleneglycol, hexamethylene glycol, and the like; mono(meth)acrylates anddi(meth)acrylates of triol compound such as trimethylolpropane,glycerin, trimetylolethane, and the like; mono(meth)acrylates,di(meth)acrylates and tri(meth)acrylates of tetra- or more valent polyolsuch as pentaerythritol, dipentaerythritol, and the like; acrylic typecompounds containing hydroxy group such as, glycerin monoallyl ether,glycerin diallyl ether and the like are suitable. These acrylic typedouble bonds need to exist at least one and preferably 2 to 20 in thebinder molecule.

[0047] The polyurethane acrylate resin is generally obtained by areaction of a resin containing hydroxy group(B′) and an acrylic typecompound containing hydroxyl group(A′) and a compound containingpolyisocyanate(C′).

[0048] Examples of the resin containing hydroxy group includepolyalkylene glycols such as polyethylene glycol, polybutylene glycol,polypropylene glycol, and the like, alkylene oxide adduct of bisphenolA, polyether polyols (B′) which has various kinds of glycols andhydroxyl groups at the terminal of the molecular chain. Of these, apolyurethane acrylate resin obtained by using polyether polyol (B′) asone component is preferable.

[0049] Examples of carboxylic acid component of polyether polyol(B′)include aromatic dicarboxylic acids such as terephthalic acid,isophthalic acid, orthophthalic acid, 1, 5-naphthalic acid, and thelike, aromatic oxycarboxylic acids such as p-oxybenzoic acid,p-(hydroxyethoxy)benzoic acid, and the like, aliphatic dicarboxylicacids such as succinic acid, adipic acid, azelaic acid, sebacic acid,dodecanoic dicarboxylic acid, and the like, unsaturated aliphatic acidsand alicyclic dicarboxylic acids such as fumaric acid, maleic acid,itaconic acid, tetrahydrophthalic acid, hexahydrophthalic acid, and thelike, tri- or tetra-carboxylic acids such as trimellitic acid, trimesicacid, pyromellitic acid, and the like.

[0050] Examples of glycol component of the polyether polyol (B′) includeethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol,1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol,diproylene glycol, 2,2, 4-trimethyl-1, 3-pentanediol,1,4-cyclohexanedimethanol, ethylene oxide adducts and propylene oxideadducts of bisphenol A, etc., ethylene oxide and propylene oxide adductof hydrogenated bisphenol A, polyethylene glycol, polypropylene glycol,polytetramethylene glycol, and the like. Also, tri- or tetra-ols such astrimethylolethane, trimetylolpropane, glycerin, pentaerythritol, and thelike may be used in combination.

[0051] Examples of polyether polyol include, in addition to the examplesmentioned above, lactone type polyesterdiol chain obtained by ringopening polymerization of lactone such as caprolactone.

[0052] Examples of polyisocyanate (C′) used include diisocyanatecompounds such as 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate,p-phenylene diisocyanate, biphenylmethane diisocyanate, m-phenylenediisocyanate, hexamethylene diisocyanate, tetramethylene diisocyanate,3,3′-dimethoxy-4, 4′-biphenylene diisocyanate, 2,4-naphthalenediisocyanate, 3,3′-demethyl-4,4′-biphenylene diisocyanate,4,4′-diphenylene diisocyanate, 4,4′-diisocyanate-diphenyl ether,1,5-naphthalene diisocyanate, p-xylylene diisocyanate, m-xylylenediisocyanate, 1,3-diisocyanate methylcyclohexane, 1,4-isocyanatemethylcyclohexane, 4,4′-diisocyanate dicyclohexane, 4,4′-diisocyanatecyclohexylmethane, isophorone diisocyanate, and the like, ortriisocyanate compounds such as 2,4-tolylene diisocyanate-trimer,hexamethylene dilsocyanate-trimer of 7 or lower mol % in all isocyanategroups, and the like.

[0053] Furthermore, in order to improve dispersibility, it is preferableto introduce polar groups such as —SO₄Y, —SO₃Y, —POY, —PO₂Y, —PO₃Y,—COOY (Y represents H or alkaline metal), —SR, —NR₂, —N⁺R₃Cl⁻(Rrepresents H or hydrocarbon group), phosphobetaine, sulfobetaine,phosphamine, sulfamine, and the like into the copolymer by optionalmethods as required. Also, to improve heat stability, an introduction ofepoxy group is preferable.

[0054] On the other hand, apart from the above synthetic method of theradiation curing type urethane, modification of thermosetting typepolyurethane resin as raw material to radiation functional type resinmay be carried out, in the same method as the vinyl chloride type resin.

[0055] Also, a radiation curing type monomer or an oligomer may be usedas required, and the use of them enables the coating layer to have ahigher crosslinking degree. The additional content is preferably 30parts by weight or less, and more preferably 20 parts by weight or lesswith respect to the resin contained in the lower non-magnetic layercoating material. When the content exceeds 30 parts by weight, thecoating material is given a strong influence and this conversely resultsin a lower gloss. The radiation curing type monomer or oligomer may beadded either after preparing the coating material or the time ofdispersing.

[0056] The content of the radiation curing binder in the lowernon-magnetic layer is preferably 10 to 100 parts by weight, and morepreferably 12.5 to 70 parts by weight with respect to 100 parts byweight of the total of carbon black and organic powders. When thecontent of the binder is excessively small, the ratio of binder resin inthe lower non-magnetic layer lowers and sufficient strength of thecoating layer is unable to be obtained. When the content of the binderis excessively large, dispersion failure occurs at the time of preparingthe lower non-magnetic layer coating material and a smooth surface ofthe lower non-magnetic layer is unable to be formed.

[0057] Examples of radiation used in the present invention include theelectron beam, γ ray, β ray, ultraviolet ray, etc., and the preferableone is the electron beam. The irradiation dose is preferably 1 to 10Mrad and more preferably 3 to 7 Mrad. The irradiation energy(acceleration voltage) is preferably 100 kV or more. The radiation ispreferably irradiated before winding after coating and drying, but itmay be irradiated after winding.

[0058] In the lower non-magnetic layer of the present invention, thelubricant is preferably contained as required. For the lubricant,irrespective of saturated or unsaturated, one or two kinds or more ofknown fatty acids, esters, and saccharides may be used individually ormixed. Preferably, two kinds or more of fatty acids and/or esters havingrespectively different melting points are used in combination. This isbecause the lubricant that matches any temperature environment used forthe magnetic recording medium must be continuously fed to the mediumsurface.

[0059] Specifically, for the fatty acid, saturated straight-chain fattyacids such as stearic acid, palmitic acid, myristic acid, lauric acid,erucic acid etc.; fatty acids which are saturated and have side chains,such as isocetylic acid, isostearic acid, etc.; and unsaturated fattyacids such as oleic acid, linoleic acid, linolenic acid, etc. can beused suitably. Foresters, there are straight-chain saturated fatty acidesters such as butyl stearate, butyl palmitate, etc.; saturated fattyacid esters with side chains such as isocetyl stearate, isostearylstearate etc.; unsaturated fatty acid esters such as isostearyl oleate,etc.; fatty acid esters of unsaturated alcohol such as oleyl stearate,etc.; esters of unsaturated fatty acid and unsaturated alcohol such asoleyl oleate, etc.; esters of dihydric alcohol such as ethylene glycoldistearate, etc.; esters of dihydric alcohol and unsaturated fatty acidesters such as ethylene glycol monooleate, ethylene glycol dioleate,neopentyl glycol dioleate, etc.; and esters of saturated or unsaturatedfatty acid and saccharides such as sorbitan monostearate, sorbitantristearate, sorbitan monooleate, sorbitan trioleate, etc. The contentof the lubricant of the lower non-magnetic layer may be adjustedsuitably in accord with objects, but is preferably 1 to 20% by weightwith respect to the total weight parts of carbon black and inorganicpowders.

[0060] The coating material for forming the lower non-magnetic layer isprepared by adding an organic solvent to the above-mentioned components.There is no particular limit to the organic solvent used, and one or twokinds or more of various solvents such as ketone type solvents such asmethyl ethyl ketone (MEK), methyl isobutyl ketone, cyclohexanone, etc.,or aromatic type solvents such as toluene, etc. may be suitably chosenand used. The amount of organic solvent added may be about 100 to 900parts by weight with respect to 100 parts by weight of a total amount ofsolids (carbon black and various inorganic powders, etc.) and thebinder.

[0061] The surface roughness of the lower non-magnetic layer (here, thesurface roughness was expressed by the centerline average roughness(JIS-B-0601) Ra) must be satisfactory. Ra after smoothing treatment ofthe lower non-magnetic layer is preferably 8.0 nm or lower, and morepreferably 6.0 nm or lower, and the most preferably 5.0 nm or lower.When Ra exceeds 8.0 nm, the interface between the upper magnetic layerand the lower non-magnetic layer tends to become non-uniform, and outputvariation tends to occur.

[0062] The thickness of the lower non-magnetic layer is in general 0.1to 2.5 μm, and preferably 0.3 to 2.3 μm. When the non-magnetic layer isexcessively thin, it tends to be subject to the influence of surfaceroughness of the non-magnetic support, and as a result, the surfacesmoothness of the non-magnetic layer is degraded and the surfacesmoothness of the magnetic layer tends to be degraded, and theelectromagnetic conversion characteristics tend to lower. In addition,because the light transmittance becomes higher, problems occur when thetape end is detected by the change of light transmittance. On the otherhand, increasing the thickness of the non-magnetic layer exceeding acertain level does not improve the performance.

[0063] [Upper magnetic layer]

[0064] The upper magnetic layer contains at least a ferromagneticpowder, a binder resin, and an abrasive having a Mohs hardness of 6 orhigher and a smaller average particle size than a thickness of the uppermagnetic layer.

[0065] In the present invention, for the ferromagnetic powder, metalalloy fine powder or hexagonal plate-like fine powder are desirable tobe used. Preferably the metal alloy fine powder has a coercive force Hcof 1500 to 3000 Oe, a saturation magnetization σ s of 120 to 160 emu/g,an average major axis length of 0.05 to 0.2 μm, an average minor axislength of 10 to 20 nm, and an aspect ratio of 1.2 to 20. Hc of themedium fabricated is preferably 1500 to3000 Oe. For additive elements,Ni, Zn, Co, Al, Si, Y and other rare earth elements etc. may be added inaccord with purposes. Preferably the hexagonal plate-like fine powderhas a coercive force Ha of 1000 to 2000 Oe, a saturation magnetization σs of 50 to 70 emu/g, an average flake particle diameter of 30 to 80 nm,and a plate ratio of 3 to 7. Hc of the medium fabricated is preferably1200 to 2200 Oe. For additive elements, Ni, Co, Ti, Zn, Sn and otherrare earth elements etc. may be added in accord with purposes. For othermaterials, known materials may be used in accord with purposes withoutparticular limitation.

[0066] The above ferromagnetic powder may be contained by about 70 to 90parts by weight in the magnetic layer composition. When the content ofthe ferromagnetic powder is excessively large, the content of the binderreduces and the surface smoothness by calendering tends to be degraded,whereas when the content of the ferromagnetic powder is excessivelysmall, no high reproduction output is able to be obtained.

[0067] For the binder resin, there is no particular limit to the resinused if it is generally used, and any of thermoplastic resin,thermosetting or reactive type resin, and radiation curing type binderresin may be used.

[0068] Examples include polyester-polyurethane resin, vinyl chloridetype copolymer, vinyl chloride-acrylate type copolymer, vinylchloride-vinyl acetate type copolymer, vinyl chloride-vinylidenechloride type copolymer, vinyl chloride-acrylonitrile type copolymer,acrylate-acrylonitrile type copolymer, acrylate-vinylidene chloride typecopolymer, methacrylate-vinylidene chloride type copolymer,methacrylate-ethylene type copolymer, poly(vinyl fluoride)-vinylidenechloride-acrylonitrile type copolymer, acrylonitrile-butadiene typecopolymer, polyamide resin, poly(vinyl butyral), cellulose derivatives(cellulose acetate butyrate, cellulose diacetate, cellulose triacetate,cellulose propionate, nitrocellulose, etc.), styrene-butadiene typecopolymer, polyester resin-chlorovinylether acrylate type copolymer,amino resin, and synthetic rubber thermoplastic resins etc.

[0069] The content of the binder used for the magnetic layer is 5 to 40parts by weight, and particularly preferably 10 to 30 parts by weightwith respect to 100 parts by weight of ferromagnetic powders. When thecontent of the binder is excessively small, the strength of the magneticlayer lowers, and the running durability tends to be degraded. On theother hand, when the content of the binder is excessively large, thecontent of ferromagnetic powders lowers and the electromagneticconversion characteristics tend to be degraded.

[0070] For a crosslinking agent for curing these binders, variouspolyisocyanates, particularly, diisocyanate can be used, andparticularly, one or more kinds of tolylene diisocyanate, hexamethylenediisocyanate, methylene diisocyanate are preferably used. Thecrosslinking agent is particularly preferably used as crosslinking agentmodified by compound having two or more hydroxyl groups such astrimethylolpropane, etc. or isocyanurate type crosslinking agentconsisting of three molecules of diisocyanate compound, and is bonded tofunctional groups, etc. contained in the binder resin and crosslink theresin. The content of the crosslinking agent is preferably 10 to 30parts by weight with respect to 100 parts by weight of the binder. Suchthermosetting resins may be cured by heating in an oven at 50 to 70° C.for 12 to 48 hours in general.

[0071] In addition, in the magnetic layer, the abrasive having the Mohshardness of 6 or higher and smaller average particle size than thethickness of the upper magnetic layer is contained in order to improvemechanical strength of the magnetic layer and to prevent clogging of themagnetic head.

[0072] Examples of the abrasive include α-alumina (Mohs hardness:9),chromium oxide (Mohs hardness:9), silicon carbide (Mohs hardness:9.5),silicon oxide (Mohs hardness 7), aluminum nitride (Mohs hardness : 9),boron nitride (Mohs hardness:9.5), etc. Of these abrasives, it ispreferable that at least one kind of abrasives with a Mohs hardness of 9or higher are contained. These are, in general, amorphous, preventclogging of the magnetic head, and improve the strength of the coatinglayer.

[0073] It is necessary for the average particle size of the abrasive tobe smaller than the thickness of the upper magnetic layer. The averageparticle size of the abrasive is, for example, 0.01 to 0.2 μm, andpreferably 0.05 to 0.2 μm although it depends on the thickness of theupper magnetic layer. When the average particle size is excessivelylarge, the projections from the magnetic layer surface increase, andthereby, degradation of electromagnetic conversion characteristics,increase of drop-out and increase of head wear etc. tend to occur. Whenthe average particle size is excessively small, the projections from themagnetic layer surface decrease and effects of preventing head cloggingbecome insufficient.

[0074] Also, for the abrasive, it is preferable that two or more kindsof abrasives which have different average particle sizes to each otherare contained. Furthermore, decrease of output variations andimprovement of running durability are hopefully obtained.

[0075] The average particle size is, in general, measured by atransmission electron microscope. The content of the abrasive ispreferably 3 to 25 parts by weight, and more preferably 5 to 20 parts byweight with respect to 100 parts by weight of the ferromagnetic powders.

[0076] In addition, in the magnetic layer, a dispersing agent such assurfactant, etc., a lubricant such as higher fatty acid, fatty acidester, silicon oil, etc., and other various additives may be added asrequired.

[0077] The coating material for forming the magnetic layer may beprepared by adding organic solvents to each component mentioned above.There is no particular limitation to the organic solvent used andorganic solvents similar to those used for the lower non-magnetic layermay be used.

[0078] The thickness of the upper magnetic layer is 0.30 μm or less, andpreferably 0.05 to 0.30 μm, and more preferably 0.10 to 0.25 μm. Thethickness of less than 0.20 μm, for example 0.15 μm, is also preferable.When the magnetic layer is excessively thick, self-magnetization lossand thickness loss increase.

[0079] The centerline average roughness (Ra) of the magnetic layersurface is preferably 1.0 to 8.0 nm, and more preferably 2.0 to 7.0 nm.When Ra is less than 1.0 nm, the surface is excessively smooth, andthereby the running stability is degraded, and troubles during runningtend to occur frequently. On the other hand, when Ra exceeds 8.0 nm, themagnetic layer surface becomes rough, and thereby electromagneticconversion characteristics such as reproduced output, etc. tend to bedegraded.

[0080] [Back-coat layer]

[0081] The back-coat layer is provided for improving the runningstability and preventing electrification of the magnetic layer, etc. Theback-coat layer preferably contains 30 to 80% by weight of carbon black.When the carbon black content is excessively small, the electrificationpreventing effect tends to lower, and in addition, the running stabilitytends to lower. Also the light transmittance of the medium tends toincrease, and thereby problems occur in the system for detecting thetape end by the change of light transmittance. On the other hand, whenthe carbon black content is excessively large, the strength of theback-coat layer lowers, and the running durability tends to degrade. Anykind of carbon black may be used if it is used in general, and theaverage particle size is preferably about 5 to 500 nm. The averageparticle size is, in general, measured by a transmission type electronmicrograph.

[0082] In the back-coat layer, besides the carbon black, non-magneticinorganic powders such as various abrasives, etc. mentioned above indescribing the magnetic layer may be contained in order to improve themechanical strength. The content of non-magnetic inorganic powder ispreferably 0. 1 to 5 parts by weight, and more preferably 0. 5 to 2parts by weight with respect to 100 parts by weight of carbon black. Theaverage particle size of non-magnetic inorganic powder is preferably 0.1to 0.5 μm. When the content of this kind of non-magnetic inorganicpowder is excessively small, the mechanical strength of the back-coatlayer tends to become insufficient, while if the content is excessivelygreat, the wear rate of guides, etc. in the tape sliding passage tendsto increase.

[0083] In addition to these, a dispersing agent such as surfactant, etc., a lubricant such as higher fatty acid, fatty acid ester, silicon oil,etc. , and other various additives may be added as required.

[0084] The binder, crosslinking agent, solvent, etc. used for theback-coat layer may be the same as those used for the coating materialfor the magnetic layer mentioned above. The content of the binder ispreferably 15 to 200 parts by weight, and more preferably 50 to 180parts by weight with respect to 100 parts by weight of the total solidsexcept the binder. When the binder content is excessively large,friction with the medium sliding passage becomes excessively large, therunning stability lowers, and hence running accidents tend to occurfrequently. In addition, problems such as blocking with the magneticlayer, etc. tend to occur. When the binder content is excessively small,the strength of the back-coat layer lowers and the running durabilitytends to lower.

[0085] The thickness of the back-coat layer (after calendering) is 1.0μm or less, and preferably 0.1 to 1.0 μm, and more preferably 0.2 to 0.8μm. When the back-coat layer is excessively thick, friction with themedium sliding passage becomes too large, and hence the runningstability tends to lower. On the other hand, when the back-coat layer isexcessively thin, the surface smoothness of the back-coat layer lowersdue to influence of the surface roughness of the non-magnetic support.Consequently, when the back-coat is thermo-cured, the surface roughnessof the back-coat layer is transferred to the magnetic layer surface,resulting in lowering of the output in high-frequencies, S/N and C/N.When the back-coat layer is excessively thin, abrasion of the back-coatlayer occurs during the running of the medium.

[0086] [Non-magnetic support]

[0087] There is no particular limitation to materials used for thenon-magnetic support. The material of the non-magnetic support may beselected from various flexible materials and various rigid materialscorresponding to the object and made into a given shape and size such asa tape form corresponding to various standards. Examples of the flexiblematerial include various type resins, for example, polyesters such aspolyethylene terephthalate and polyethylene naphthalate, polyolefinssuch as polypropylene, polyamides, polyimides and polycarbonates.

[0088] The thickness of the non-magnetic support is preferably 3.0 to75.0 μm. The shape of the non-magnetic support may be, though notparticularly limited to, a tape form, sheet form, card form or disk formand the like. Various materials may be selected and used according tothe shape and requirements.

[0089] The surface roughness of the non-magnetic support used in thepresent invention is 20 nm or lower, and preferably 15 nm or lower interms of the centerline average surface roughness Ra. The surfaceroughness of the non-magnetic support is freely controlled by the sizeand the amount of filler added to the non-magnetic support as required.Examples of the filler include organic resin fine powder such as acrylictype resin in addition to oxides or carbonates of Ca, Si, T, Al and thelike, and a combination of Al₂O₃ and organic resin fine powder ispreferable.

[0090] [Manufacturing process]

[0091] The magnetic recording medium of the present invention may bemanufactured by preparing the lower non-magnetic layer coating materialand the upper magnetic layer coating material, respectively, using theabove-mentioned materials, and applying the coating materials on thenon-magnetic support in this order.

[0092] Each coating material for the lower non-magnetic layer and theupper magnetic layer is manufactured by at least a kneading step and adispersing step, as well as by a mixing step, viscosity adjusting stepand filtering step before and/or after these steps, as required. Eachstep may be divided into two or more stages. Any of the materials suchas ferromagnetic powders, non-magnetic inorganic powders, binders,abrasives, carbon blacks, lubricants, solvents, etc. may be added at thebeginning or in the midway of any step. In addition, individual materialmay be divided and added in two or more steps.

[0093] For kneading and dispersing of the coating material, theconventionally known techniques may, of course, be used in part or wholeof the processes, but in the kneading step, any machine having strongkneading force such as a continuous kneader or a pressure kneader ispreferably used. When the continuous kneader or the pressure kneader isused, the ferromagnetic powder or the non-magnetic inorganic powder andall or part of the binder (however, preferably 10% or more by weight ofall the binder) are kneaded. The slurry temperature at the time ofkneading is preferably 50° C. to 110° C.

[0094] For dispersing the coating material, it is desirable to use adispersing media of high specific gravity, and ceramic type media suchas zirconia, titania, etc. are suitable. Glass beads, metal beads,alumina beads, etc. which have been conventionally used may be selectedand used depending on the composition.

[0095] In the manufacturing method for the magnetic recording medium ofthe present invention, the lower non-magnetic layer coating material isapplied onto the non-magnetic support and dried, and resulting layer issubjected to smoothing treatment and cured by irradiating with radiationto obtain the lower non-magnetic layer, and then the upper magneticlayer coating material is applied onto said lower non-magnetic layer.

[0096] The back-coat layer may be formed before or after orsimultaneously with formation the lower non-magnetic layer and the uppermagnetic layer.

[0097] For the coating means, any of, for example, gravure coat, reversecoat, extrusion nozzle, etc. may be used, but the method for using dienozzle coater is preferable from the viewpoint of operability andproductivity.

[0098] After applying the lower non-magnetic layer coating material, thecoating material may be dried at drying temperature of room temperatureto about 300° C. The smoothing treatment may be carried out like asmoothing treatment of the magnetic layer mentioned below. After thesmoothing treatment, the non-magnetic layer cured by irradiating withradiation is obtained.

[0099] Subsequently, the upper magnetic layer coating material mentionedabove is applied onto the lower non-magnetic layer.

[0100] In the present invention, in the case of magnetic tapes, it ispreferable to orient magnetic particles in the magnetic layer byapplying the magnetic field after the magnetic layer is formed. Theorientation direction may be parallel or vertical or slanting withrespect to the running direction of the medium in accord with purposes.In order to orient to the specified direction, it is preferable to applythe magnetic field 1000G or more with permanent magnet such as ferritemagnet or rare earth magnet, etc., electromagnet, solenoid, etc. or touse a plurality of these magnetic field generating means in combination.Furthermore, to achieve the highest orientation after drying, a suitabledrying step may be effected before orientation or orientation is carriedout simultaneously with drying. In the case of floppy disks, magneticpowders naturally oriented by coating may be brought to thenon-orientation condition as much as possible with permanent magnet,electromagnet, solenoid, etc.

[0101] The magnetic coating layer that has undergone the orientationtreatment after coating in this way is, in general, dried and fixed byknown drying and evaporation means such as hot air, far infrared rays,electric heater, vacuum equipment, etc. equipped inside the dryingfurnace. The drying temperature is in the range from room temperature toabout 300° C., and may be suitably selected in accord with the heatresistance of the non-magnetic support, kind and/or concentration ofsolvent, etc., and temperature gradient may be provided inside thedrying furnace. In addition, the gas atmosphere inside the dryingfurnace may be, in general, air or inert gas.

[0102] After drying the magnetic layer in this way, calendering iscarried out for surface smoothing treatment as required. For thecalendering roll, a combination of heat-resistant plastic rolls such asepoxy, polyester, nylon, polyimide, polyamide, polyimideamide, etc.(rolls with carbon, metal, or other inorganic compounds kneaded may beused) and metal rolls (three to seven-high combination) may be used.Also, a combination of metal rolls only may be used. The treatmenttemperature is preferably 90° C. or higher, and more preferably 100° C.or higher. The linear pressure is preferably 200 kg/cm or higher, andmore preferably 250 kg/cm or higher, and the treatment speed is in therange from 20 m/min to 900 m/min. In the present invention, furthereffects can be obtained by the treatment temperature of 100° C. orhigher and the linear pressure of 250 kg/cm or higher.

EXAMPLES

[0103] The present invention will be described in more detail referringto embodiments, but the present invention should not be limited to theseembodiments.

[0104] [Preparation of upper magnetic layer coating material (a)]

[0105] (Preparation of binder solution) Vinyl chloride type resin 10parts by weight (MR-110, manufactured by Nippon Zeon Co., Ltd.)Polyester polyurethane resin 7 parts by weight (UR-8300, manufactured byToyobo Co., Ltd.) MEK (methyl ethyl ketone) 21 parts by weight Toluene21 parts by weight Cyclohexanone 21 parts by weight

[0106] The above compositions were charged into a hyper mixer and mixedwith stirring to give a binder solution.

[0107] (Kneading)

[0108] The following compositions were charged into a pressure kneaderand kneaded for 2 hours. α-Fe magnetic powder 100 parts by weight (Hc =1650 Oe, σs = 126 emu/g, BET = 57 m²/g, major axis length = 0.10 μm)α-Al₂O₃ 2 parts by weight (HIT-60A, manufactured by Sumitomo ChemicalCo., Ltd.; average particle size = 0.20 μm) α-Al₂O₃ 10 parts by weight(HIT-82, manufactured by Sumitomo Chemical Co., Ltd.; average particlesize = 0.13 μm) The binder solution 40 parts by weight

[0109] The following compositions were added to the slurry afterkneading, and the slurry was adjusted to an optimum viscosity fordispersing treatment. The binder solution 40 parts by weight MEK 15parts by weight Toluene 15 parts by weight Cyclohexanone 15 parts byweight

[0110] (Dispersion)

[0111] The slurry was dispersed by a sand mill.

[0112] (Viscosity adjusting solution)

[0113] The following compositions were charged into a hyper mixer andmixed with stirring for one hour to give a viscosity adjusting solution.The viscosity adjusting solution was circulation-filtered by using adepth filter with 95% cut filtration accuracy of 1.2 μm. Stearic acid0.5 part by weight Myristic acid 0.5 part by weight Butyl stearate 0.5part by weight MEK 210 parts by weight Toluene 210 parts by weightCyclohexanone 210 parts by weight

[0114] (Viscosity adjustment)

[0115] After the above solution and the dispersed slurry were mixed withstirring, the mixture was subjected to dispersion treatment again by asand mill to give a coating material. The coating material wascirculation-filtered by using a depth filter with 95% cut filtrationaccuracy of 1.2 μm.

[0116] (Final coating material)

[0117] To 100 parts by weight of the coating material after filtration,0.8 part by weight of an isocyanate compound (Coronate-L, manufacturedby Nippon Polyurethane) was added, and the mixture was mixed withstirring and circulation-filtered by using a depth filter with 95% cutfiltration accuracy of 1.2 μm to give a final coating material (a) formagnetic layer.

[0118] [Preparation of upper magnetic layer coating materials (b)-(j)]

[0119] Upper magnetic layer coating materials (b)-(j) were prepared inthe same manner as in the above preparation of the upper magnetic layercoating material (a) except that the followings were respectively usedas the abrasive in place of a total of 12 parts by weight of α-Al₂O₃.(b) α-Al₂O₃ 12 parts by weight (HIT-60A, manufactured by SumitomoChemical Co., Ltd.; average particle size = 0.20 μm) (c) α-Al₂O₃ 12parts by weight (HIT-82, manufactured by Sumitomo Chemical Co., Ltd.,average particle size = 0.13 μm) (d) α-Al₂O₃ 12 parts by weight(AKP-100, manufactured by Sumitomo Chemical Co., Ltd.; average particlesize = 0.06 μm) (e) α-Al₂O₃ 12 parts by weight (HIT-100, manufactured bySumitomo Chemical Co., Ltd.; average particle size = 0.04 μm) (f)α-Al₂O₃ 2 parts by weight (HIT-82, manufactured by Sumitomo ChemicalCo., Ltd.; average particle size = 0.13 μm) α-Al₂O₃ 10 parts by weight(AKP-100, manufactured by Sumitomo Chemical Co., Ltd.; average particlesize = 0.06 μm) (g) α-Al₂O₃ 2 parts by weight (HIT-82, manufactured bySumitomo Chemical Co., Ltd.; average particle size = 0.13 μm) α-Al₂O₃ 10parts by weight (HIT-100, manufactured by Sumitomo Chemical Co., Ltd.;average particle size = 0.04 μm) (h) α-Al₂O₃ 12 parts by weight (AKP-20,manufactured by Sumitomo Chemical Co., Ltd.; average particle size =0.50 μm) (i) α-Al₂O₃ 12 parts by weight (E-700, manufactured by NortonCo., Ltd.; average particle size = 0.30 μm) (j) chromium oxide 12 partsby weight (S-1, manufactured by Nippon Chemical Industrial Ltd.; averageparticle size = 0.40 μm)

[0120] [Preparation of lower non-magnetic layer coating material (k)]

[0121] (Preparation of binder solution) Electron radiation curing typevinyl chloride 10 parts by weight type resin (Vinyl chloride-epoxycontaining monomer copolymer; average degree of polymerization = 310;epoxy content = 3 wt %; S content = 0.6 wt %; acryl content = 6 groups/1molecule; Tg = 60° C.) Electron radiation curing type polyester 7 partsby weight polyurethane resin (Phosphorus compound-hydroxy containingpolyester polyurethane; number average molecular weight = 13000; acrylcontent = 6 groups/1 molecule; Tg = 10° C.) MEK 21 parts by weightToluene 21 parts by weight Cyclohexanone 21 parts by weight

[0122] The above compositions were charged into a hyper mixer andstirred to give a binder solution.

[0123] (Kneading)

[0124] The following compositions were charged in a pressure kneader andkneaded for 2 hours. Needle-form α-Fe₂O₃ 10 parts by weight (DPN-250BWmanufactured by Toda Kogyo Corp.; major axis length = 0.15 μm; specificsurface area = 53 m²/g) Carbon black 90 parts by weight (Raven 760Bmanufactured by Columbian Chemicals Company; average particle size = 30nm; specific surface area = 70 m²/g; DPB oil absorption = 48 mL/100 g)The binder solution 40 parts by weight

[0125] The following compositions were added to the slurry afterkneading, and the slurry was adjusted to an optimum viscosity fordispersing treatment. The binder solution 40 parts by weight MEK 15parts by weight Toluene 15 parts by weight Cyclohexanone 15 parts byweight

[0126] (Dispersion)

[0127] The slurry was dispersed by a sand mill.

[0128] (Viscosity adjusting solution)

[0129] The following compositions were charged into a hyper mixer, andstirred to give a viscosity adjusting solution. Stearic acid 0.5 part byweight Myristic acid 0.5 part by weight Butyl stearate 0.5 part byweight MEK 100 parts by weight Toluene 100 parts by weight Cyclohexanone100 parts by weight

[0130] (Viscosity adjustment and final coating material)

[0131] After the above solution and the dispersed slurry were mixed withstirring, the mixture was subjected to dispersion treatment again by asand mill to give a coating material. The coating material wascirculation-filtered by using a depth filter with 95% cut filtrationaccuracy of 1.2 μm to give a final coating material (k) for lowernon-magnetic layer.

[0132] [Preparation of lower non-magnetic layer coating material (l)]

[0133] (Preparation of binder solution) Vinyl chloride type resin 10parts by weight (MR-110, manufactured by Nippon Zeon Co., Ltd.)Polyester polyurethane resin 7 parts by weight (UR-8300, manufactured byToyobo Co., Ltd.) MEK 21 parts by weight Toluene 21 parts by weightCyclohexanone 21 parts by weight

[0134] The above compositions were charged into a hyper mixer andstirred to give a binder solution.

[0135] (Kneading)

[0136] The following compositions were charged in a pressure kneader andkneaded for 2 hours. Needle-form α-Fe₂O₃ 10 parts by weight (DPN-250BWmanufactured by Toda Kogyo Corp.; major axis length = 0.15 μm; specificsurface area = 53 m²/g) Carbon black 90 parts by weight (Raven 760Bmanufactured by Columbian Chemicals Company; average particle size = 30nm; specific surface area = 70 m²/g; DPB oil absorption = 48 mL/100 g)The binder solution 40 parts by weight

[0137] The following compositions were added to the slurry afterkneading, and the slurry was adjusted to an optimum viscosity fordispersing treatment. The binder solution 40 parts by weight MEK 15parts by weight Toluene 15 parts by weight Cyclohexanone 15 parts byweight

[0138] (Dispersion)

[0139] The slurry was dispersed by a sand mill.

[0140] (Viscosity adjusting solution)

[0141] The following compositions were charged into a hyper mixer, andstirred to give a viscosity adjusting solution. Stearic acid 0.5 part byweight Myristic acid 0.5 part by weight Butyl stearate 0.5 part byweight MEK 100 parts by weight Toluene 100 parts by weight Cyclohexanone100 parts by weight

[0142] (Viscosity adjustment)

[0143] After the above solution and the dispersed slurry were mixed withstirring, the mixture was subjected to dispersion treatment again by asand mill to give a coating material. The coating material wascirculation-filtered by using a depth filter with 95% cut filtrationaccuracy of 1.2 μm.

[0144] (Final coating material)

[0145] To 100 parts by weight of the coating material after filtration,0.8 part by weight of an isocyanate compound (Coronate-L, manufacturedby Nippon Polyurethane) was added, and the mixture was mixed withstirring and circulation-filtered by using a depth filter with 95% cutfiltration accuracy of 1.2 μm to give a final coating material (l) forlower non-magnetic layer.

[0146] [Preparation of back-coat layer coating material]

[0147] (Preparation of binder solution) Vinyl chloride-vinylacetate-vinyl alcohol  65 parts by weight copolymer (Monomer weightratio = 92:3:5; average degree of polymerization = 420) polyesterpolyurethane resin  35 parts by weight (UR-8300, manufactured by ToyoboCo., Ltd.) MEK 260 parts by weight Toluene 260 parts by weightCyclohexanone 260 parts by weight

[0148] The above compositions were charged into a hyper mixer andstirred to give a binder solution.

[0149] (Dispersion)

[0150] The following compositions were charged in a ball mill anddispersed for 24 hours. Carbon black 80 parts by weight (Conductex SC,manufactured by Columbian Chemicals Company; average particle size = 20nm, BET = 220 m²/g) Carbon black 1 part by weight (Sevacarb MT,manufactured by Columbian Chemicals Company; average particle size = 350nm, BET = 8 m²/g) α-Fe₂O₃ 1 part by weight (TF100, manufactured by TodaKogyo; average particle size = 0.1 μm) The binder solution 880 parts byweight

[0151] (Viscosity adjusting solution)

[0152] The following compositions were charged into a hyper mixer, andstirred to give a viscosity adjusting solution. Stearic acid 1 part byweight Myristic acid 1 part by weight Butyl stearate 2 parts by weightMEK 210 parts by weight Toluene 210 parts by weight Cyclohexanone 210parts by weight

[0153] (Viscosity adjustment)

[0154] After the above solution and the dispersed slurry were mixed withstirring, the mixture was subjected to dispersion treatment again by aball mill for 3 hours. The resulting coating material wascirculation-filtered by using a depth filter with 95% cut filtrationaccuracy of 1.2 μm.

[0155] (Final coating material)

[0156] To 100 parts by weight of the coating material after filtration,1 part by weight of an isocyanate compound (Coronate-L manufactured byNippon Polyurethane) was added, and the mixture was mixed with stirringand circulation-filtered by using a depth filter with 95% cut filtrationaccuracy of 1.2 μm to give a coating material for back-coat layer.

[0157] [Preparation of magnetic tape]

[0158] (Examples 1-10, Comparative Examples 1-4)

[0159] In Examples 1-10 and Comparative Examples 1-4, coating materialsfor the upper magnetic layer and the lower non-magnetic layer shown inTables 1 and 2 were used.

[0160] The lower non-magnetic layer coating material was applied onto asurface of a non-magnetic support (polyethylene terephthalate film witha thickness of 8.3 μm) and dried, and resulting layer was calendered andirradiated with electron radiation in nitrogen gas atmosphere to obtaincured layer. Onto the lower non-magnetic layer, the upper magnetic layercoating material was applied, followed by orientation treatment, dryingand calendering. Each thickness of the upper magnetic layer and thelower non-magnetic layer after calendering is respectively shown inTables 1 and 2. Further, the back-coat layer coating material wasapplied onto the back surface of the non-magnetic support. After drying,calendering was carried out. In all samples, each thickness of theback-coat layer after calendaring was 0.5 μm.

[0161] After resulting roll was let to stand at ordinary roomtemperature for 24 hours, the roll was cured at 60° C. in a heating ovenfor 24 hours, and then slit down to a width of 8 mm and incorporated ina cassette to give a magnetic tape sample.

[0162] (Comparative Example 5)

[0163] The lower non-magnetic layer coating material (l) was appliedonto a surface of a non-magnetic support (polyethylene terephthalatefilm with a thickness of 8.3 μm), and then while the lower non-magneticlayer coating material was in a wet state, the upper magnetic layercoating material (a) was applied thereto, followed by orientationtreatment, drying and calendering. Each thickness of the upper magneticlayer and the lower non-magnetic layer after calendering is shown inTable 2. Further, the back-coat layer coating material was applied ontothe back surface of the non-magnetic support. After drying, calenderingwas carried out. A thickness of the back-coat layer after calendaringwas 0.5 μm.

[0164] After resulting roll was let to stand at ordinary roomtemperature for 24 hours, the roll was cured at 60° C. in a heating ovenfor 24 hours, and then slit down to a width of 8 mm and incorporated ina cassette to give a magnetic tape sample.

[0165] (Comparative Example 6)

[0166] The lower non-magnetic layer coating material (l) was appliedonto a surface of a non-magnetic support (polyethylene terephthalatefilm with a thickness of 8.3 μm) and dried, followed by calendaring androlling up. After resulting roll was let to stand at ordinary roomtemperature for 24 hours, the roll was cured at 60° C. in a heating ovenfor 24 hours. The upper magnetic layer coating material (a) was appliedonto the lower non-magnetic layer, followed by orientation treatment,drying and calendering. Each thickness of the upper magnetic layer andthe lower non-magnetic layer after calendering is shown in Table 2.Further, the back-coat layer coating material was applied onto the backsurface of the non-magnetic support. After drying, calendaring wascarried out. A thickness of the back-coat layer after calendaring was0.5 μm.

[0167] After resulting roll was let to stand at ordinary roomtemperature for 24 hours, the roll was cured at 60° C. in a heating ovenfor 24 hours, and then slit down to a width of 8 mm and incorporated incassette to give a magnetic tape sample.

[0168] The following is evaluation for each magnetic tape sampleobtained.

[0169] (Output Variation)

[0170] By using a spectrum analyzer (Advantest:TR4171), RF output at thetime when a sine wave signal of Frequency 750 kHz was recorded andreproduced by an optimum recording current was measured. The spectrumanalyzer was set to a center Frequency at 750 kHz, a Frequency span at 0Hz and a sweep time at optimum value, and an output variation: Vp-p (dB)was confirmed.

[0171] (Running Durability)

[0172] In an environment of 20° C. and 60% RH, VTR reciprocating 100running passes were carried out for 50 tape sample cassettes, andoccurrence of running troubles such as running-stop, head clogging andthe like was judged by standards below. The deck used: EV-S900 (Hi8format VTR) manufactured by Sony.

[0173] ⊚: No troubles occurred at all.

[0174] ◯: Trouble occurred in one cassette.

[0175] X : Troubles occurred in two or more cassettes.

[0176] (Head wear)

[0177] In an environment of 20° C. and 60% RH, a head projection levelof before and after running for 100 hours was measured by an opticalcomparator, and the difference was represented as head wear (μm).

[0178] (Surface roughness Ra)

[0179] By using a feeler type surface-configuration instrument, TALYSTEPsystem, manufactured by Tayler-Hobson Corp., measured values wereobtained by a method described in JIS B-0601. Measurement conditionswere filter condition: 0.18 to 9 Hz, needle pressure: 2 mg, needle used:special stylus of 0.1×2.5 μm, scan speed: 0.03 mm/sec, and scan length:500 μm. Ra (nm) was obtained by the result. TABLE 1 Comparative Example1 Example 2 Example 3 Example 4 Example 5 Example 1 Example 6 Example 7Upper magnetic layer a b c d e b c d coating material Lower non-magnetick k k k k k k k layer coating material Thickness of upper  0.25  0.25 0.25  0.25  0.25  0.15  0.15  0.15 magnetic layer (μm) Particle size ofupper 0.20/0.13  0.20  0.13  0.06  0.04  0.20  0.13  0.06 layer abrasive(μm) Thickness of lower 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 non-magneticlayer (μm) Coating method W/D W/D W/D W/D W/D W/D W/D W/D Outputvariation 0.1 0.4 0.5 0.4 0.4 0.5 0.5 0.6 Vp-p (dB) Running durability ⊚◯ ◯ ◯ ◯ ⊚ ◯ ◯ Head wear (μm) 0.5 0.5 0.3 0.2 0.2 3.0 0.6 0.5 Ra (nm) 4.54.3 4.0 3.8 3.7 5.2 4.4 4.2

[0180] TABLE 2 Comparative Comparative Comparative ComparativeComparative Example 8 Example 9 Example 10 Example 2 Example 3 Example 4Example 5 Example 6 Upper magnetic layer e f g h i j a a coatingmaterial Lower non-magnetic k k k k k k l l layer coating materialThickness of upper  0.15  0.15  0.15  0.25  0.25  0.25  0.25  0.25magnetic layer (μm) Particle size of upper  0.04 0.13/0.06 0.13/0.04 0.50  0.30  0.40  0.20/0.13 0.20/0.13 layer abrasive (μm) Thickness oflower 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 non-magnetic layer (μm) Coatingmethod W/D W/D W/D W/D W/D W/D W/W W/D Output variation 0.7 0.1 0.2 0.40.3 0.7 3.0 2.5 Vp-p (dB) Running durability ◯ ⊚ ⊚ ⊚ ⊚ ⊚ × ⊚ Head ware(μm) 0.3 0.6 0.7 7.0 5.0 8.0 0.1 0.5 Ra (nm) 3.9 4.4 4.3 6.5 6.0 6.5 3.84.8

[0181] In the coating methods shown in the Tables, W/D representswet-on-dry coating method and W/W represents wet-on-wet coating method.

[0182] As shown in Tables 1 and 2, each of the tape sample in Examples1-10 did not occur output variations, had a small head wear, and had anexcellent running durability. By using two kinds of abrasives havingdifferent average particle sizes to each other, we find that outputvaritations lowered further and running durability was improved stillmore.

[0183] On the other hand, in Comparative Examples 1-4, the averageparticle size of the abrasive was larger than the thickness of the uppermagnetic layer and the head wear became larger. In Comparative Example5, which was an example of a case that the upper magnetic layer wasapplied while the lower non-magnetic layer was in a wet state withoutusing the radiation curing type binder resin for the lower non-magneticlayer, the interface between the upper magnetic layer and the lowernon-magnetic layer became non-uniform and this resulted in outputvariations, and also, the running durability was degraded. InComparative Example 6, which was an example of a case that the uppermagnetic layer was applied onto the lower non-magnetic coating layerafter thermosetting the lower coating layer, a tightened winding of thelower layer occurred, and coating of the upper magnetic layer wasnon-uniform resulting in output variations.

[0184] As this invention may be embodied in several forms withoutdeparting from the spirit or essential characteristics thereof, theforegoing examples are therefore only illustrative and should not beinterpreted as restrictive, and all changes that fall within equivalenceof claims are therefore intended to be embraced by the claims.

What is claimed is:
 1. A magnetic recording medium comprising a lowernon-magnetic layer containing at least a carbon black and a radiationcuring type binder resin on a non-magnetic support and an upper magneticlayer having a thickness of 0.30 μm or less on the lower non-magneticlayer, wherein the upper magnetic layer contains at least aferromagnetic powder, a binder resin, and an abrasive having a Mohshardness of 6 or higher and a smaller average particle size than thethickness of the upper magnetic layer.
 2. The magnetic recording mediumaccording to claim 1, wherein the thickness of the upper magnetic layeris 0.05 to 0.30 μm.
 3. The magnetic recording medium according to claim1, wherein the average particle size of the abrasive is 0.01 to 0.2 μm.4. The magnetic recording medium according to claim 1, wherein acenterline average roughness (Ra) of the upper magnetic layer surface is1.0 nm≦Ra≦8.0 nm.
 5. The magnetic recording medium according to claim 1,wherein the abrasive contains two or more kinds of abrasives which havedifferent average particle sizes to each other.
 6. A process forproducing a magnetic recording medium which comprises: preparingrespectively a lower non-magnetic layer coating material including atleast a carbon black dispersed into a radiation curing type binderresin, and an upper magnetic layer coating material including at least aferromagnetic powder, and an abrasive having a Mohs hardness of 6 orhigher and a smaller average particle size than a thickness of an uppermagnetic layer to be formed into a binder resin, applying the lowernon-magnetic layer coating material onto a non-magnetic support, dryingthe coating material, and carrying out smoothing treatment of andirradiating with radiation to resulting layer to form a lowernon-magnetic layer, and then applying the upper magnetic layer coatingmaterial onto the lower non-magnetic layer, drying the coating material,and carrying out smoothing treatment of resulting layer to form an uppermagnetic layer.
 7. The process for producing the magnetic recordingmedium according to claim 6, wherein an orientation treatment is carriedout after the upper magnetic layer coating material is applied.
 8. Themagnetic recording medium according to claim 1 which is produced by theprocess according to claim 6 or 7.